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Physical Network Considerations

The propagation delay occurs in the Network Interface layer 1. This is where light passes through fiber, and electric current flows through Ethernet (etc.). Internet networks consist of a variety of mixed forms of physical point-to-point

TCP/IP and OSI network layer models compared

Figure 2.2 TCP/IP and OSI network layer models compared.

links terminated by various telecommunications and networking systems that then interface with each other at junctions called routers. The computational processing in the terminations usually, but not always, happens as part of an electrical process or a photoelectric process with little computed logic - so, for example, fiber multiplexers operate optically based on wavelength of light, and Ethernet switches work electrically based on the sequence of bits in the header of the datagram. There may be some delay introduced, but at this stage the latency is largely affected by propagation delay more than any other form of delay.

Internet Layer Considerations

Once the packets are handed to the router - the Internet layer 2 - things start to change. The router must have a degree of “intelligence” to decide which of the multiple possible “other connected network links” that router should forward each packet. The first time a stream of data is routed, this forwarding path must be discovered, and this can take a few milliseconds - possibly more. This adds latency to the time it takes these packet to reach the recipient.

Once the route is discovered, the router will make a note and will store that routing in a local “directory” called a “routing table” This minimizes the processing that the router must do. However, any changes to the networks condition will cause a “re-evaluation” of the route over which the router can forward packets. Again, this will add some latency to the packets that are being throughput.

Routing is typically an optimized process and uses dedicated technology that can function by introducing minimal latency.

It is important to understand the effects of the network layer in introducing propagation. When streaming live, the variety of options one is presented with along the workflow can indeed critically affect the overall process.

If the original signal is delivered over a network link that is prone to varied propagation delay or other contributing latency factors, then no matter how well constructed the onward distribution network is, it can only distribute a varied and delayed source image or sound.

 
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